In his first-ever interview, the sole Australian survivor of the Waco siege has revealed to 60 Minutes what really took place during the 51-day stand-off, and why he believes crazed cult leader David Koresh will one day return to Earth as the son of God.

Angry protesters take to the streets of Barcelona over independence vote

Thousands took to the streets of Barcelona on Wednesday as Spanish police detained 13 Catalan government officials in a crackdown ahead of an independence referendum which Madrid says is illegal.

With tensions mounting, separatist organisations called for more people to protest as leaders in the northeastern region pressed ahead with preparations for the October 1 vote despite Madrid's ban and a court ruling deeming it unconstitutional.

Spanish Prime Minister Mariano Rajoy called for calm, urging "a return to normality and a return to common sense because the referendum cannot take place."

But Catalan President Carles Puigdemont accused Madrid of imposing a "de facto" state of emergency to try and stop the referendum.

Among those arrested by the Guardia Civil police was Josep Maria Jove, secretary general of economic affairs and Catalonia's deputy vice president, a regional government spokesman said.

The others work in various Catalan government departments, including its economic and budget affairs departments, a local Guardia Civil spokesman said.

The reason for the arrests was not immediately clear, but Spain's central government has warned that officials who help stage the referendum could face criminal charges.

An influential pro-independence citizens' organisation, the Catalan National Assembly (ANC), urged Catalans to hit the streets in protest.

"Let's go out to defend our institutions in a non-violent way. They made a big mistake, we wanted to vote and they declared war," its president Jordi Sanchez said in a statement.

The police operation comes a day after officers seized a trove of documents related to the independence referendum from the offices of Unipost, a private delivery company, in Terrassa near Barcelona.

Police said they confiscated over 45,000 notifications which were about to be sent to Catalans selected to staff polling stations for the vote, representing 80 percent of the numbers necessary to ensure the stations were adequately staffed.

Police scuffled with dozens of pro-secession protesters who gathered outside Unipost's office to keep officers from entering the building.

A demonstrator holds a banner translating "We Vote To Be Free" within a protest in front of the Economy headquarters of Catalonia's regional government in Barcelona. (AFP)

Madrid has also threatened to arrest mayors who facilitate the vote, has seized posters and fliers that promote the plebiscite and has tightened its control over the region's finances.

It says the constitution stipulates that a Spanish region does not have the right to call a referendum.

Tensions have also reached the parliament in Madrid, where Gabriel Rufian, a lawmaker for the pro-separatist Catalan Republican Left (ERC), on Wednesday told Mr Rajoy to take his "dirty hands off Catalan institutions".

"The will of the Catalan people cannot be stopped. And now we will leave to support our friends," he added before storming out of the assembly with fellow ERC lawmakers.

But Mr Rajoy defended his government's position, saying it was "fulfilling its obligation."

Pro-separatist parties captured 47.6 percent of the vote in a September 2015 Catalan election which was billed as a proxy vote on independence, giving them a narrow majority of 72 seats in the 135-seat Catalan parliament.

But opinion polls show Catalonia's roughly 7.5 million residents are deeply divided on independence.

A survey commissioned by the regional government in July showed that 49.4 percent of Catalans were against independence while 41.1 percent were in favour.

But more than 70 percent of Catalans said they wanted a referendum on independence to settle the issue.